This class represents a Response event that is passed from a SipProvider to
its SipListener. This specification handles the passing of Response messages
to the application with the event model. An application (SipListener)
registers with the SIP protocol stack (SipProvider) and listens for Response
events from the SipProvider.

This specification defines a single Response event object to handle all
Response messages. The Response event encapsulates the Response message
that can be retrieved from getResponse().
Therefore the event type of a Response event can be determined as follows:

eventType == ResponseEvent.getResponse().getStatusCode();

A Response event also encapsulates the client transaction upon which the
Response is correlated, i.e. the client transaction of the Request
message upon which this is a Response.

ResponseEvent contains the following elements:

source - the source of the event i.e. the SipProvider sending the
ResponseEvent.

clientTransaction - the client transaction this ResponseEvent is
associated with.

Response - the Response message received on the SipProvider
that needs passed to the application encapsulated in a ResponseEvent.

Gets the Dialog associated with the event or null if no dialog exists.
This method separates transaction support from dialog support. This
enables application developers to access the dialog associated to this
event without having to query the transaction associated to the event.
For example the transaction associated with the event may return 'null'
because the final response for the transaction has already been received
and the stack has no more record of the transaction. This situation can
occur when a UAC sends requests out through a forking proxy. Responses
that all refer to the same transaction may be sent by the targets of the
fork but each response may be stamped with a different To tag, thus
referring to different Dialogs on the UAC. The first final response
terminates the transaction but the UAC may want to create a Dialog on
a subsequent response.

Returns:

the dialog associated with the response event or null if there is no dialog.